1. Field
Embodiments of the present invention relate to a method for determining exposure settings, a lithographic exposure apparatus, a computer program and a data carrier.
2. Background
A lithographic exposure apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target field of the substrate. A lithographic exposure apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In a lithographic projection apparatus commonly used in the manufacture of integrated circuits, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target field (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically done by using a projection system for imaging the pattern on the patterning device onto a layer of radiation-sensitive material (resist) provided on the substrate supported by a wafer table. In general, a single substrate will contain a network of adjacent target fields that are successively patterned.
In a lithographic exposure process for a wafer known from US Patent Appl. Publ. No. 2007/0263191, incorporated herein by reference in its entirety, the position of the target field (portion) in the direction of the optical axis of the projection system is determined with a multipoint AF system arranged to measure with one row M detection points (spots) simultaneously. The different spots form a linear array with different positions in a direction X perpendicular to a scanning direction Y. These position measurements are used to bring the target fields in focus of the projection exposure system.
A wafer includes a plurality of target fields separated from each other by scribe lanes. The wafer will be cut at the scribe lanes during a later process step.
The linear array of spots forms an elongate detection area with a length set around the width of the wafer.
The detection points are each associated with a sensor of the multipoint AF sensor. The offset between the sensors of the multipoint AF sensor is calibrated by measuring the positions of a CD bar using the multipoint AF sensor and combining the measurements with Z sensors positioned on two sides of the elongate detection area. The offsets correspond to the deviation between the value measured at a certain X position (and thus a certain detection point) and the linear interpolation between the measurements of the Z-sensors.
Additionally a traverse Z-moving correction is performed. This is to avoid inaccurate positioning of the wafer table during exposure because of measurement errors made by encoders used to measure and control the position of the wafer table. During the traverse Z-movement correction, the position of the wafer table is measured using Z-sensors while detecting surface information with the multipoint AF-sensor at detection points having predetermined distances. Because the offset between sensors of the multipoint AF sensor has been calibrated earlier, when two sensors of the multipoint AF sensor measure the same point, they should obtain the same value. The differences between measurements at the same point reflect differences of the position of the wafer table (also measured with the Z-sensors) and inaccuracies. Then the Z-sensors are calibrated to compensate for these inaccuracies.
According to the method a step includes focus mapping. During focus mapping a straight line (centerline) parallel to the Y-axis that passes through the center of the wafer table (which substantially coincides with the center of the wafer) coincides with straight line LV through a plurality of encoders. The wafer table is scanned in the Y-direction controlling its position using the Z-sensors. During the scan, the multipoint AF sensor measures at predetermined sampling intervals.
Finally, the focus map is used to determine a position during exposure, the wafer table is scanned by keeping it under Z-sensor control after relating the measurements from the focus mapping with the best focus position of the projection system.
As said earlier the detection points (spots) form a linear array with different positions in a direction X perpendicular to the scanning direction Y. This is to prevent cross talk between the detection points. Between the detection points the position of the target area may differ for the position at the detection points themselves. This means that between the detection points it is not possible to determine the best focus position, which consequently leads to a lower overall focus control for the target field.